The present invention relates to magnetic resonance imaging, and more particularly to methods for detecting the presence of a ferromagnetic object within a medical patient to be imaged.
Magnetic resonance imaging (MRI) is a common modality used in medical diagnosis. An MRI system utilizes very strong magnetic fields to cause precession of the magnetic moments of the nuclei of atoms within the medical patient. The magnetic moments are aligned by radio frequency magnetic fields and the decay of such alignment is detected. The nuclei of different atoms decay at various rates, thereby enabling use of the detected decay characteristics of different regions within the patient to create an image of internal organs.
Numerous surgical procedures permanently insert metallic objects, such as ocular implants, heart valve protheses, stents and orthopedic implants. Magnetic resonance imaging may be contraindicated for an individual with implanted ferromagnetic material, because of the risk associated with movement or dislodgement of the implanted object. In addition, other possible hazards exist, including the induction of electric currents in the ferromagnetic object, excessive heating and misinterpreting an image artifact caused by the object as an abnormality.
In the case of surgically implanted ferromagnetic materials, the patient usually knows of their existence and can communicate that fact to the physician or technician performing the magnetic resonance imaging. However, the presence of such surgical implants may not be known in situations where the patient is unconscious or otherwise cannot communicate with the medical personnel, as often occurs with accident victims. In addition, patients may not be aware of the presence of other ferromagnetic objects, such as a metal shard which accidentally became embedded in the patient's body. Therefore, a patient who is to undergo magnetic resonance imaging may not know or be able to inform the medical personnel about the presence of internal ferromagnetic objects.
Previous procedures for screening medical patients prior to MRI presumed that the patient was aware of both the presence and precise nature of the ferromagnetic object. Such information was used to look up the implanted object in a table that also listed the magnetic properties to different types of devices which could be present within a patient. A table of this type is given in an article entitled, "MR Procedures and Biomedical Implants, Materials, and Devices: 1993 Update," Radiology, November 1993, pages 587-599. However, such screening methods can not be used in situations where the patient is unaware of an object's presence or exact type.
Therefore, it is desirable to provide a method for not only detecting the presence of ferromagnetic material within a medical patient, but the ferromagnetic properties of the object.
Conventional metal detectors, such as those used to locate objects under the sand on a beach, cannot be utilized to detect relatively small ferromagnetic objects, such as a metal shard. Even such small objects can create serious problems during MRI due to the very intense magnetic fields employed. Furthermore, common metal detectors determine the presence of a metallic object by sensing high electrical conductivity. Because the primary hazards during MRI relate to the magnetic characteristics of implants, conductivity based detection does not accurately portray the hazard potential during imaging. It should be noted that the conductivity of an object does not indicate the magnetic characteristics.